refactor(tests): one definition each for the copied fixtures and fakes (#2825, milestone 296 area 1)

The shape ledger showed the same test scaffolding defined over and over:
_bind_user x12 (byte-identical), _dispose_engine x10 in three wordings,
_no_supersession x3, _make_mock_session x7 in three subsets, a get-or-create
User helper x2 (+3 inlined), and fifteen hand-rolled MagicMock note factories
each re-explaining the same "an auto-MagicMock attribute is truthy" hazard
(note 2109).

Now: conftest.py carries _bind_user / _dispose_engine / _no_supersession as
opt-in fixtures (pytestmark = usefixtures(...) per module, so unit tests pay
nothing), and tests/helpers.py carries make_mock_session(), ensure_user() and
fake_note(**attrs) — the hazard documented once, real values on every
attribute the product reads. Call sites were rewritten by AST so titles with
dashes and commas survived; the three SimpleNamespace _note stand-ins that
only feed a single function stay local.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_snippet_provenance.py

@@ -24,6 +24,8 @@ a DB-touching path with only mocked coverage is a path with no coverage).
 import pytest
 import pytest_asyncio
 
+from tests.helpers import ensure_user
+
 from scribe.services.snippets import (
     VERIFY_CHANGED,
     VERIFY_OK,
@@ -79,34 +81,13 @@ def test_verification_records_and_reads_back_the_checked_commit():
 
 # --- integration: the rules through the real service paths -------------------
 
-@pytest_asyncio.fixture
-async def _dispose_engine():
-    from scribe.models import engine
-    yield
-    await engine.dispose()
-
 
 @pytest_asyncio.fixture
 async def user_id(_dispose_engine):
-    # Get-or-create: the lane's database persists across tests, so a second
-    # test re-creating the same username dies on the unique constraint.
-    from sqlalchemy import select
-
     from scribe.models import async_session
-    from scribe.models.user import User
 
     async with async_session() as s:
-        existing = (
-            await s.execute(
-                select(User).where(User.username == "snippet_prov_itest")
-            )
-        ).scalar_one_or_none()
-        if existing is not None:
-            return existing.id
-        user = User(username="snippet_prov_itest")
-        s.add(user)
-        await s.flush()
-        uid = user.id
+        uid = (await ensure_user(s, "snippet_prov_itest")).id
         await s.commit()
     return uid